Structural Instability of Si(111)-(7x7) Induced by Low-Energy Electron Irradiation

1. Structural Instability of Si(111)-(7x7) Induced by Low-Energy Electron Irradiation Tanimura laboratory Yoshiaki Sugita

2. 0.015 N-type 0.01 0.005 3.51x10-6 ML/pulse DENSITY OF VACANCIES (ML) 0 P-type 0.01 0.005 1.46x10-6 ML/pulse 0 0 1000 2000 3000 4000 NUMBER OF LASER PULSES Background DAS model Si(111) ideal surface Semiconductor Surfaceｓ unfaulted half Reconstruction and/or relaxation →quasi two-dimensional system →surface specific phenomena corner adatom faulted half center adatom Laser-Induced Electronic Bond Breaking 500-nm laser pulse Before irradiation After irradiation 600nm 35mJ/cm2 ・ Bond breaking via an electronic process. ・ Superlinear with respect to the excitation intensity. ・ Fermi-level effect in bond breaking efficiency N-type > P-type. J. Kanasaki. et al. Phy. Rev. Lett. 80, 4080 (1998) Multiple localization of surface holes describe the results reasonably.

3. 14.5 5 7.5 1.9 d2N/dE2 (arb. units) Si(111)-(7x7) 4 8 20 12 16 0 ENERGY LOSS (eV) Purpose Electron-induced structural instability Electrons or holes from STM tips A. Kobayashi, F. Grey, R. S. Williams, M. Aono, Science 259, 19(1993) Sample Bias What are characteristic features? Mechanisms? Electron Energy Loss Spectrum Laser individual excitation only Electron Beam Ep=100eV individual excitation ＋ Plasmon excitation Low-Energy Electron Irradiation →　 Direct imaging of structural changes of Si(111)-(7x7) by means of scanning tunneling microscope Bulk Plasmon（16.9） Surface Plasmon（10.2） ・　Identification of excitation mode ・　electronic mechanism? J. E. Rowe and H. Ibach, Phys. Rev. Lett. 31, 102(1973)

4. (a) before irradiation (b) 60 sec (a) 1.5V (before) (b) -2.4V (before) mono vacancy (c) 120sec (d) 180sec (c) 1.5V (60sec) (d) -2.4V (60sec) vacancy cluster STM Images of Structural Changes Primary Energy : 15eV Electron Beam Flux : 1.54x1012[cm-2s-1] Dark spots produced by irradiation Independent of sample bias No adsorbates Bond breaking at adatom sites → formation of isolated vacancies Irradiation time increases Density of vacancies increases. Structural instability induced by electron irradiation.

5. Primary Energy : 15eV Electron Beam Flux : 1.54x1012[cm-2s-1] 0.06 0.05 0.04 SUM v1 0.03 NUMBER DENSITY [ML] v2 0.02 v3 v4 0.01 0 0 60 120 180 IRRADIATIONTIME [sec] Size distribution of vacancies Monovacancy is the primary product Bond breaking at intrinsic surface sites is the main process.

6. Primary Energy 15eV Primary Energy 15eV 5 0.06 0.05 4 0.04 3 Density of Vacancies [ML] 0.03 Ratio (center/corner) 2 0.02 1 0.01 ○　1.54x1012[cm-2s-1] ○　1.54x1012[cm-2s-1] 0 100 200 300 400 100 200 300 400 0 Irradiation Time [sec] IRRADIATION TIME [sec] Dose and Site Dependence ・Center adatoms are removed 3 times more efficiently than corner adatoms. ・Independent of irradiation dose, electron beam flux and primary energy linear slope efficiency Common features of electron-induced vacancy formation

7. 3 2 1 0 2 4 6 8 Flux Dependence of the Efficiency EFFICIENCY OF VACANCY FORMATION [x10-4ML s-1] ●15eV ●20eV ELECTRON BEAM FLUX [x1014cm-2s-1] linear with respect to the electron beam flux Linear slope cross section strong energy dependence

8. Energy Dependence of Cross Section 10 8 6 EFFICIENCY [x10-17ML cm2] 4 2 0 0 10 20 30 40 50 PRIMARY ENERGY [eV] Rapidly decaying from 15 to 50eV

9. Comparison Morphologic features of structural change are the same for both excitation sources. The last process of the bond breaking is common. Multiple localization of surface holes is important also in the case of Electron-induced bond breaking. Why linear?

10. = K N R d Two Hole Localization (THL) Model H.Sumi,Surf.Sci.248,382(1991). hole R － Nd － hole hole THLprobability: Superlinear with respectto the excitation intensity individual excitation case (laser excitation)

11. Evaluation of hole density needed for THL • s ； cross section • ； flux N；density of state • t ； lifetime of hole Electron Beam Excitation Laser Excitation wavelength500nm The magnitude of a can be estimated from the inverse of mean free path. （≒5x10-7[cm]） Photon Energy≒2.48[eV] Absorption coefficienta=1.85x104[cm-1] laser fluence30mJ/cm-2 Electron Beam Fluxf≒1013cm-2s-1 f≒1x1025[cm-2s-1] Hole density Hole density In the electron irradiation, individual excitation could not produce THL states.

12. Plasmon Excitation Lifetime of Plasmon τ～10fs AMPLITUDE OF SURFACE PLASMON t 0 IRRADIATION TIME Multiple excitation of plasmon is not possible. THL can be induced by a single plasmon. Localization probability : P Excitation rate of plasmon per unit time: Efficiency of Bond Breaking σp：excitation probability of plasmon φ：electron beam flux Linear with respect to beam flux This model can describe the observed results .

13. Energy dependence of Surface Plasmon Energy Loss Probability surface plasmon generation probability vs. primary energy 10 8 kbp 6 EFFICIENCY [x10-17 ML cm2] 4 ksp 2 0 10 20 30 40 50 PRIMARY ENERGY [eV] The plasmon-related bond breaking model can describe qualitativelythe results.

14. Summary ・　Structural instability induced by low-energy electron irradiation. Local bond breaking at intrinsic surface sites by electronic mechanism Monovacancy is the primary product. strongly site-dependent yields （center/corner = 3 ） similarly to the case of laser excitation Multiple localizations of the surface hole is important in case of the bond breaking by the electron beam. Efficiency of bond breaking contrast to the case of laser excitation Linear with respect to excitation intensity Strong energy dependence Individual excitation mode is not responsible for the bond breaking. Surface Plasmon model possibly describe the experimental results.